1. Field of the Invention
The present invention generally relates to a MOS type power semiconductor switching device with a device protection against load short-circuit problems. More specifically, the present invention is directed to a MOS (metal-oxide semiconductor) type power switching device capable of preventing under low heat dissipation the MOS type power switching device from being heated and possibly destroyed due to an occurrence of an extraordinary condition such as a short-circuit in the load.
2. Description of the Prior Art
Various MOS types of power semiconductor switching devices have been proposed in the semiconductor device field. For instance, Japanese Laid-open (KOKAI DISCLOSURE) patent application No. 64-68005 opened in 1989 discloses a power MOSFET switching device 7 with a load protection function. An internal circuit of this power MOSFET switching device is shown in FIG. 1A and an I-V (current-to-voltage) characteristic thereof is presented in FIG. 1B. As indicated in FIG. 1A, this power MOSFET switching device 7 is constructed of a power MOSFET "FET.sub.1 ", a MOSFET "FET.sub.2 ", and a bipolar transistor "Tr.sub.1 ". The typical operation of this power MOSFET switching device 7 is performed as follows. Power consumption within the chip of the power MOSFET "FET.sub.1 " is detected based on a combination of the drain-to-source voltage "V.sub.DS " of this MOSFET and the drain current "I.sub.D ", when the detected power consumption reaches a predetermined condition, the gate-to-source voltage "V.sub.GS " is turned OFF thereby turning OFF this power MOSFET "FET.sub.1 ". As a result, the power MOSFET switching device 7 can be continuously operated within the area of safety operation (abbreviated "ASO") of this power MOSFET (see FIG. 1B).
As previously explained, since the above-described conventional power MOSFET switching device 7 is continuously operated within the ASO region thereof, this MOSFET switching device 7 may not be electrically destroyed due to over power consumption. As a result, the MOSFET switching device 7 may be prevented from being over-heated due to an occurrence of extraordinary load condition, and thereafter electrically destroyed.
In FIG. 2, there is shown one example in which the above-described power MOSFET switching device 7 is employed to drive a load "R.sub.L ". FIG. 2A shows a circuit diagram of this load drive circuit and FIG. 2B represents an I-V characteristic of the power MOSFET switching device 7. As shown in FIG. 2A, the drain of this power MOSFET switching device 7 is connected via the load R.sub.L to a power supply (not shown) at a voltage of "V.sub.DD ", a source of this switching device 7 is grounded, and a gate thereof is connected via a resistor "R.sub.IN " to a gate drive voltage "V.sub.IN ".
When an extraordinary condition, e.g., a short-circuit happens to occur in the load R.sub.L, since the voltage V.sub.DD of the power supply is directly applied to the drain of this power MOSFET switching device 7, the operating point of this circuit is removed to a point "O", as shown in FIG. 2B However, this new operating point "O" is located within the ASO region indicated by a dot line of FIG. 2B, so that this power MOSFET switching device 7 is effectively protected.
The above-described power MOSFET switching device 7 with the load protection function has the following problems.
First, as apparent from the circuit diagram shown in FIG. 1A, since the drain "D" of the power MOSFET "FET.sub.1 " is connected via a series circuit or resistors R.sub.1 and R.sub.2 to the source "S" thereof, even when this MOSFET "FET.sub.1 " is turned OFF by applying no gate voltage, a current "I.sub.2 " may flow from this drain "D" via the resistors R.sub.1 and R.sub.2 to the source "S". As a result, a large leak current may be present under such a condition that the power MOSFET switching device 7 is turned OFF, resulting in high power consumption.
Another conventional power MOSFET switching device utilized in a current sensing circuit is described in U.S. Pat. No. 4,553,084 to Wrathall, issued on Nov. 12, 1985. This conventional circuit is so-called "current mirror type load protection circuit".
FIG. 3 indicates this current mirror type load protection circuit, and FIG. 4 partially indicates an I-V characteristic of a power MOSFET switching device 10A employed in this current mirror type load protection circuit (see a curve "20"). As easily understood from FIG. 4, the drain current of the power MOSFET switching device 10A is limited to a peak drain current "I.sub.D-P " when a load 12 is short-circuited in this current mirror type protection circuit.
However, such a conventional current mirror type protection circuit has the following problems. That is, since power consumption of the power MOSFET 10A is a product of a current flowing through this MOSFET 10A and an applied voltage thereof, when the applied voltage becomes high under a condition when the current value is a constant, the resultant power consumption becomes large. Thus, an area of safety operation represents such a characteristic curve whose right portion is lowered. As previously explained, since the current value becomes constant at the peak current value "I.sub.D-P " (See FIG. 4), if the load 12 is short-circuited, the voltage V.sub.DD of the power supply is directly applied to the power MOSFET switching device 10A. As a consequence, the power consumption "P" of the power MOSFET 10A under short-circuited load condition is defined by: EQU P=I.sub.D-P .times.V.sub.DD.
The resultant power consumption "P" is of rather high value, and therefore high heat dissipation may occur. In other words, since this power MOSFET switching device must be designed to be safely operated even with such a high heat condition (namely, ASO region should be broadened), the dimensions of this switching device should be made large and a degree of freedom is designing heat dissipation would be lowered.